Power electronic converters including semiconductor valves are widely used to change the character of electrical energy by means of controlling the voltage, current and/or frequency. Power electronic converters are for example increasingly used in power transmission applications, such as in HVDC transmission systems or in static VAR compensation systems for utility or industry grids.
In such applications, it is important that the down time of the converters is reduced to a minimum, since failure of a converter may result in power outages, which can be costly for society as well as for the power provider. Despite efforts to improve the life time and cycling capability of electronic components, fault free operation of such components cannot be guaranteed. Hence, redundancy is an important concept when designing power transmission systems.
Multi-level power electronic converters having a plurality of cells which are series connected in a cascade fashion have been proposed in DE 101 03 031, as well as in “A multilevel Voltage-Source Inverter with Separate DC Sources for Static Var Generation”, Fang Zheng Peng et al., IEEE IAS Conf 1995 Proc. Cascaded converters provide a plurality of discrete converter output voltage levels, thereby facilitating synthesizing of a sinusoidal voltage waveform. Moreover, such cascaded converters inherently provide redundancy at low expense in that the terminals of the faulty cell can be short-circuited and the faulty cell by-passed, while the remaining cells can continue to operate normally. Thus, despite a faulty cell, the remaining cells can continue to deliver the desired voltage as long as the remaining number of cells is sufficient.
Different mechanisms for short-circuiting the terminals of semiconductor devices have been proposed, see for example WO 2007/023064, wherein a protection component is connected in parallel to a rectifier circuit, in order to provide short-circuiting of the terminals of the rectifier circuit in case of a fault; or WO 2007/095873, wherein a pyrotechnical/mechanical element is used in a short-circuiting operation. These mechanisms require additional hardware, and thus increase the cost and volume of the semiconductor devices.
In “Fault-Tolerant Transformerless Power Flow Controller Based-on ETO Light Converter”, Wenchao Song et al., Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition, 2008, a cascaded multilevel converter having H-bridge building block cells is disclosed. It is suggested that when a top/bottom switch of the H-bridge is failed short, another top/bottom switch is turned on, while the two complementary switches are turned off, thus making the H-bridge building block cell enter a shorted state. This method of obtaining an AC short circuit between the cell terminals can be implemented without any additional hardware. However, the mechanism requires that power is provided to the gate drive unit of another top/bottom switch in order to keep this switch turned on. In case the cell is shorted out completely, no DC voltage will be available to the cell, and the power required to maintain another top/bottom switch in the turned on state cannot easily be provided within the cell.